Electrical machines, especially rotating electrical machines, typically contain an electrical winding within a laminated core. The winding comprises electrical leads (possibly already provided with primary insulation) and solid insulating materials as their principal insulation. Without further measures, there is no compact connection between the laminated core, the leads, and the principal insulation, leaving gaps and voids. On operation under atmospheric conditions, these regions are filled with air. Particularly with applications in the high-voltage range, this is unallowable, since partial electrical discharges would destroy the insulation within a very short time. This leads to the failure of the electrical machine.
To avoid gaps and voids, the winding is generally impregnated with an impregnating resin or impregnating varnish. The impregnating resin fills the voids and gaps and penetrates, where there is porosity, into the solid porous materials. Certain solid insulating materials such as mica tapes, insulating papers, or nonwoven webs are made extra-porous, to absorb the impregnating resin. In the prior art, mixtures of epoxy resins and liquid, cycloaliphatic acid anhydrides are used for the impregnation. The acid anhydrides here serve as hardeners and at the same time lower the viscosity, which is beneficial for rapid and complete impregnation. Using the liquid, cycloaliphatic acid anhydrides requires safety measures.